1. Field of the Invention
The present invention relates generally to wireless data networks, and specifically to scheduling access of traffic flows to the wireless communication medium.
2. Background of the Invention
Wireless local area networks (WLAN) are being used for convenient interconnection of portable computers to office data processing networks and to the Internet. Presently there is a desire to increase WLAN data throughput and quality of service in order to support applications such as voice over Internet Protocol (IP) telephony, and real-time video streaming. For example, the Institute of Electrical and Electronics Engineers (IEEE) of Washington, D.C., is promulgating a standard 802.11 n for WLAN to provide data throughputs of at least 100 Mbit/sec.
One way of increasing the WLAN data throughput while providing an acceptable quality of service is to use the bandwidth of the wireless transmission medium more efficiently. Because user stations in the WLAN share access to the wireless transmission medium, a considerable amount of the existing bandwidth is consumed in a process of coordinating access of the traffic flows to the wireless transmission medium. In general, the nature of the wireless transmission medium precludes user stations from being synchronized to the extent that interference between user stations is unlikely. Therefore, collision avoidance techniques are generally used in the access control process to the wireless transmission medium.
IEEE standard 802.11 e provides a framework to provide acceptable quality of service as perceived by traffic flows accessing the wireless channel. Traffic flows can be serviced using either of two medium access methods, depending on the flow's preference. The first access method, called Hybrid Coordination function Channel Access (HCCA), is polling based. In this first method, a Hybrid Coordination Function (HCF) grants transmission opportunities (TXOP) to all participating flows based on a schedule. The second access method, called Enhanced Distributed coordination Channel Access (EDCA), is contention-based, in which flows compete to access the channel. In this second method, flows transmit packets whenever they sense that the shared medium is idle for a sufficient period of time. The Hybrid Coordination Function determines when each HCCA flow gets a chance to transmit and when control is ceded to EDCA traffic.
IEEE standard 802.11 e does not specify a particular method that the HCF must use for selecting a participating flow to receive a next TXOP. There are a number of known methods that the HCF could use, such as Earliest Deadline First (EDF), and Weighted Round Robin (WRR).
EDF scheduling is usually used to schedule real-time events with deadline constraints. EDF schedules the event with the closest deadline. In the case of packet transmissions, EDF grants transmission to the packet whose deadline is the earliest
WRR is a scheduling discipline that allocates access to traffic flows, in a round robin fashion. Each flow receives access to the channel proportional to the flow's assigned weight. Weights are generally assigned by the network administrator.